The application of digital positioning systems in the motion control servo systems of the machine tool and automatic equipment industry is a demonstrated success. Over the years, however, many new areas of technology have developed at a very rapid pace such as robotics, IC manufacturing, and advanced instrumentation, which requires advanced positioning systems with critical performance parameters not obtained with the prior art positioning servo systems. Some critical performance characteristics such as positioning accuracy, accurate velocity and acceleration control and system response time determines the suitability of any positioning system to a given servo application.
In spite of the increasing demand for better performing servo systems with superior performance characteristics, the basic digital positioning systems have not improved for a long period of time.
The basic approach taken to solve a digital positioning problem in the prior art systems is described in Automotive Components Handbook published by Intel Corp. (1987) pages 11-7 to 11-11 inclusive. This approach represents the state of the prior art relating to the digital positioning systems. As described in this Handbook, a rotating chopper disc controlled by a DC motor is positioned by digital control means at a selected position or count by the digital sensors activated by two light sources. As described in the approach, the disc has six position markers of counts where the disc can be positioned. It has two zones or phases of markers for each count or position the chopper disc can be positioned at.
In the above example the outer track is defined as phase B and the inner track is defined as phase A. Those skilled in the art will recognize the use of quadrature output which provides information on both direction and speed of rotation of the chopper disc. The logic state of the sensor detecting the position of the counts on phase B is in quadrature phase relation to that of the sensor detecting the position of the count on phase A. The waveforms generated by the sensors for both clockwise and anticlockwise rotation are shown in the Intel Handbook.
The positioning of the chopper disc at any of the six given counts is achieved by zero crossing of the error signal. The zero error occurs at the edge transition of the logic state defining the position of the selected count or marker. It is therefore defined as lock on edge type positioning system.
Let us assume that the disc is rotating in a clockwise direction and the output logic state of the sensor is true or `1` when the sensor is covered by the count preventing the light source from activating the sensor. And, the logic state is false or "0" when the sensor is not covered by the count allowing the light to activate the sensor.
The positioning at any count is achieved when the sensor detecting the position of the counts on phase A remains at logic `1` state and sensor detecting the position of the counts on phase B changes logic state from logic "0" to "1". This occurs at the edge of the transition of logic state waveform. Or, when the error signal reaches zero value and changes sign at the edge of the signal waveform.
An ideal situation would occur if the encoder disc will be able to attain a stable position at zero value of the error signal just before the error signal changes the sign. Those skilled in the art will recognize the fact that in actual practice, however, due to finite inertia of the disc and also due to finite time delay from sensor signal to motor movement, actual and absolute positioning of the disc at the edge is not possible. The disc will rotate past the edge, which will be detected by the control logic and the direction of rotation will be reversed to lock the encoder disc at edge from "0" to "1". A constant repetitive movement in foreword and reverse direction as close to the edge as can be made possible by given system parameters will be the final position achievable with this type of positioning system. Those skilled in the art will immediately recognize the limitations and disadvantages of this system.
First, the system never stops oscillating about the final count to reach a steady logic state, and in addition, the error will vary with the change in load inertia which implies that the positioning accuracy will change with the change in the load inertia. These factors will constrain the error to never approaches to a "0" value. Secondly, regardless of how good a system is designed or critical parameter chosen, there will always exist a window of finite error around which the system stability will dwell. This will prevent an absolute positioning to be achieved. In some applications not requiring critical accuracy, that does not create any problems. And, in some applications, the system can be designed around these limitations with limited success. In high precision systems, requiring critical performance parameters, however, these limitations are not acceptable. But, in absence of availability of any suitable alternate systems in the prior art, the older systems, although not perfect and providing inferior performance, are still being used with accompanying limitations and disadvantages.
Another problem, as yet not recognized, nevertheless very important, is a need for two tracks of counts in prior art systems. This will force the disc, for a given circumference to accommodate specific number of elements to give a selected resolution on the encoder disc. Thus, for any given count, a circumferential length has to accommodate two elements or graduations defining only one position. This adds to the size, weight, and cost of the system. Another problem experienced with the prior art systems relates to the number of counts required per disc to achieve the required positioning accuracy regardless of the resolution to which measurements are necessary. The final positioning in a lock on edge type system is maintained by oscillating the disc about any given count so that larger the number of count, the smaller the oscillation of the disc and higher the positioning accuracy. As an example, for a disc having 10 counts per revolution, the resolution will be 36 deg. per count. However, the disc can not be allowed to oscillate 36 deg. because the positioning will then be very inaccurate. Therefore, to achieve higher accuracy, a very large number of counts are required. This again adds to the size, weight, and cost of the system.
As an additional approach to further improve the positioning accuracy, the article further goes on to describe a need for an additional analog circuit, employing additional components to achieve finer, more accurate positioning. There is however, a significant additional cost involved and all analog systems are subject to temperature and voltage drifts which adversely affects the final accuracy. An additional approach suggests using triangular waveform instead of using square waveforms. This approach provides better accuracy, however, still the error does not tend to approach a zero value.
In view of the problems and disadvantages of the prior art positioning systems discussed in the foregoing, it will indeed be desirable to have a positioning system which will overcome the problems, limitations and disadvantages of the prior art systems and provide desired performance with attendant cost, size and weight advantages so much desired in the cost competitive motion control technology.